Chapter 30 Plant Diversity II: The Evolution of Seed Plant

Overview: Transforming the World Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems Seed plants originated about 360 million years ago A seed consists of an embryo and nutrients surrounded by a protective coat Domestication of seed plants had begun by 8,000 years ago and allowed for permanent settlements

Living seed plants can be divided into two clades: gymnosperms and angiosperms Gymnosperms appear early in the fossil record about 305 million years ago and dominated Mesozoic (251–65 million years ago) terrestrial ecosystems Gymnosperms were better suited than nonvascular plants to drier conditions

Angiosperms began to replace gymnosperms near the end of the Mesozoic Angiosperms now dominate more terrestrial ecosystems Today, cone-bearing gymnosperms called conifers dominate in the northern latitudes

The gymnosperms consist of four phyla Cycadophyta (cycads) Gingkophyta (one living species: Ginkgo biloba) Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia) Coniferophyta (conifers, such as pine, fir, and redwood)

Phylum Cycadophyta Individuals have large cones and palmlike leaves These thrived during the Mesozoic, but relatively few species exist today

Phylum Ginkgophyta This phylum consists of a single living species, Ginkgo biloba It has a high tolerance to air pollution and is a popular ornamental tree

Ginkgo biloba pollen-producing tree Figure 30.5b Figure 30.5 Exploring: Gymnosperm Diversity Ginkgo biloba leaves and fleshy seeds Ginkgo biloba pollen-producing tree 8

Ginkgo biloba pollen-producing tree Figure 30.5ba Figure 30.5 Exploring: Gymnosperm Diversity Ginkgo biloba pollen-producing tree 9

Ginkgo biloba leaves and fleshy seeds Figure 30.5bb Figure 30.5 Exploring: Gymnosperm Diversity Ginkgo biloba leaves and fleshy seeds 10

Phylum Gnetophyta This phylum comprises three genera Species vary in appearance, and some are tropical whereas others live in deserts

Ovulate cones Gnetum Welwitschia Ephedra Figure 30.5d Figure 30.5 Exploring: Gymnosperm Diversity Welwitschia Ephedra 12

Phylum Coniferophyta This phylum is by far the largest of the gymnosperm phyla Most conifers are evergreens and can carry out photosynthesis year round

Figure 30.5 Exploring: Gymnosperm Diversity Common juniper Douglas fir Figure 30.5 Exploring: Gymnosperm Diversity Sequoia European larch Wollemi pine Bristlecone pine 14

The pine tree is the sporophyte and produces sporangia in male and female cones Small cones produce microspores called pollen grains, each of which contains a male gametophyte The familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes It takes nearly three years from cone production to mature seed

Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Angiosperms are seed plants with reproductive structures called flowers and fruits They are the most widespread and diverse of all plants

Characteristics of Angiosperms All angiosperms are classified in a single phylum, Anthophyta, from the Greek anthos for flower Angiosperms have two key adaptations Flowers Fruits

Flowers The flower is an angiosperm structure specialized for sexual reproduction Many species are pollinated by insects or animals, while some species are wind-pollinated

A flower is a specialized shoot with up to four types of modified leaves Sepals, which enclose the flower Petals, which are brightly colored and attract pollinators Stamens, which produce pollen Carpels, which produce ovules

A stamen consists of a stalk called a filament, with a sac called an anther where the pollen is produced A carpel consists of an ovary at the base and a style leading up to a stigma, where pollen is received

Stigma Carpel Stamen Anther Style Filament Ovary Petal Sepal Ovule Figure 30.7 Stigma Carpel Stamen Anther Style Filament Ovary Figure 30.7 The structure of an idealized flower. Petal Sepal Ovule 21

Fruits A fruit typically consists of a mature ovary but can also include other flower parts Fruits protect seeds and aid in their dispersal Mature fruits can be either fleshy or dry

Tomato Ruby grapefruit Nectarine Hazelnut Milkweed Figure 30.8 Figure 30.8 Some variations in fruit structure. Milkweed 23

Various fruit adaptations help disperse seeds Seeds can be carried by wind, water, or animals to new locations

The Angiosperm Life Cycle The flower is composed of both male and female structures Most flowers have mechanisms to ensure cross-pollination between flowers from different plants of the same species

A pollen grain that has landed on a stigma germinates and the pollen tube of the male gametophyte grows down to the ovary The ovule is entered by a pore called the micropyle Double fertilization occurs when the pollen tube discharges two sperm into the female gametophyte within an ovule

One sperm fertilizes the egg, while the other combines with two nuclei in the central cell of the female gametophyte and initiates development of food-storing endosperm The triploid endosperm nourishes the developing embryo Within a seed, the embryo consists of a root and two seed leaves called cotyledons

Angiosperm Evolution Darwin called the origin of angiosperms an “abominable mystery” Angiosperms originated at least 140 million years ago During the late Mesozoic, the major branches of the clade diverged from their common ancestor Scientists are studying fossils, refining phylogenies, and investigating developmental patterns to resolve the mystery

Fossil Angiosperms Chinese fossils of 125-million-year-old angiosperms share some traits with living angiosperms but lack others Archaefructus sinensis, for example, has anthers and seeds but lacks petals and sepals

(a) Archaefructus sinensis, a 125- million-year-old fossil Figure 30.11 Carpel Stamen 5 cm (a) Archaefructus sinensis, a 125- million-year-old fossil Figure 30.11 An early flowering plant. (b) Artist’s reconstruction of Archaefructus sinensis 30

Angiosperm Phylogeny The ancestors of angiosperms and gymnosperms diverged about 305 million years ago Amborella and water lilies are likely descended from two of the most ancient angiosperm lineages

Developmental Patterns in Angiosperms Egg formation in the angiosperm Amborella resembles that of the gymnosperms In early angiosperms, the two integuments appear to originate separately Researchers are currently studying expression of flower development genes in gymnosperm and angiosperm species

Angiosperm Diversity Angiosperms comprise more than 250,000 living species Previously, angiosperms were divided into two main groups Monocots (one cotyledon) Dicots (two dicots) DNA studies suggest that monocots form a clade, but dicots are polyphyletic

Monocots More than one-quarter of angiosperm species are monocots

Monocots Orchid Lily Pygmy date palm Anther Stigma Ovary Filament Figure 30.13c Monocots Orchid Lily Figure 30.13 Exploring: Angiosperm Diversity Pygmy date palm Anther Stigma Ovary Filament Barley, a grass 35

Figure 30.13 Exploring: Angiosperm Diversity Monocot Characteristics Eudicot Characteristics Embryos One cotyledon Two cotyledons Leaf venation Veins usually parallel Veins usually netlike Stems Vascular tissue usually arranged in ring Vascular tissue scattered Roots Root system usually fibrous (no main root) Taproot (main root) usually present Figure 30.13 Exploring: Angiosperm Diversity Pollen Pollen grain with one opening Pollen grain with three openings Flowers Floral organs usually in multiples of three Floral organs usually in multiples of four or five 36

Evolutionary Links Between Angiosperms and Animals Animals influence the evolution of plants and vice versa For example, animal herbivory selects for plant defenses For example, interactions between pollinators and flowering plants select for mutually beneficial adaptations

Concept 30.4: Human welfare depends greatly on seed plants No group of plants is more important to human survival than seed plants Plants are key sources of food, fuel, wood products, and medicine Our reliance on seed plants makes preservation of plant diversity critical

Products from Seed Plants Most of our food comes from angiosperms Six crops (wheat, rice, maize, potatoes, cassava, and sweet potatoes) yield 80% of the calories consumed by humans Modern crops are products of relatively recent genetic change resulting from artificial selection Many seed plants provide wood Secondary compounds of seed plants are used in medicines

Table 30.1 Table 30.1 Examples of Plant-Derived Medicines 40

Threats to Plant Diversity Destruction of habitat is causing extinction of many plant species In the tropics 55,000 km2 are cleared each year At this rate, the remaining tropical forests will be eliminated in 200 years Loss of plant habitat is often accompanied by loss of the animal species that plants support

At the current rate of habitat loss, 50% of Earth’s species will become extinct within the next 100–200 years The tropical rain forests may contain undiscovered medicinal compounds